1. Field of the Invention
This invention generally relates to semiconductor processing equipment, and more specifically relates to an improved semiconductor wafer carrier for holding a semiconductor wafer during processing.
2. Description of the Related Art
Semiconductor wafers must be polished to achieve a smooth, flat finish before performing process steps that create circuitry on the wafer. This is accomplished by securing a wafer to a wafer carrier, rotating the wafer carrier, and placing a counter-rotating polishing platen in contact with the rotating wafer.
The prior art wafer carrier is fixedly mounted on a shaft, requiring the wafer carrier and polishing platen to be aligned to be perfectly parallel. The prior art wafer carrier provides vacuum pressure through the shaft, and the shaft is coupled to the wafer carrier through three O-rings to maintain vacuum integrity through the connection. The prior art wafer carrier typically has 16 holes which are coupled to the vacuum source. These holes are in a fixed configuration and serve to secure the wafer to the wafer carrier by vacuum pressure. In addition, the prior art wafer carrier is difficult to install and remove, requiring a significant amount of time to change the configuration of the wafer carrier.
An additional problem found in prior art wafer carriers is referred to by those skilled in the art as the edge effect. In general, the undesirable result of the edge effect is that an outer ring of the polished side of the wafer has non-uniform material removal, and therefore, that portion of the wafer is not usable. Of course, this wasted portion may reduce the cost effectiveness of the final product, so it is desirable to alleviate the edge effect in order to maximize the usable area of the wafer. The edge effect is caused by the force of the platen as it meets the outer edge of the wafer and the resultant oscillations of the platen against the wafer. As a result of this force, a first outermost ring portion of the outer ring is characterized by excessive material removal, or a low area on the wafer surface. A second ring portion located adjacent to the first outermost ring portion and toward the center of the wafer is characterized by reduced material removal, or a high area on the wafer surface. A third ring portion located adjacent to the second outermost ring portion and toward the center of the wafer is characterized by excessive material removal, or a low area on the wafer surface. In short, the non-uniform material removal of the outer ring has a pattern which oscillates between excessive material removal and reduced material removal. The outer most ring portion typically experiences the greatest material removal due to the higher force experienced at the physical interface between the wafer edge and the platen. The next ring portion experiencing the reduced material removal is the result of the flexibility of the platen as it springs away from the wafer surface after being compressed by the initial contact with the outer edge surface of the wafer. Subsequently, the platen springs back toward the surface of the wafer to result in the third ring portion experiencing excessive material removal. Of course, this action happens over a short period of time, but since it occurs on a recurring basis as the polishing process continues, the edge effect causes the uneven material removal to occur over an outer ring portion adjacent to the edge and on the polished surface of the wafer. Prior art wafer carriers provided no volume into which the outer portion of the wafer could be bent due to the initial force, and the subsequent oscillations of the platen, thereby resulting in the edge effect.
Therefore, there existed a need for an improved semiconductor wafer carrier and method that provides a gimballing feature which causes the wafer carrier to self-adjust during polishing, which simplifies vacuum connections between the shaft and the wafer carrier, which has a large number of vacuum holes which can be custom-configured to accommodate a large number of process variations, which is easy to install and remove, and which removes the detrimental edge effect.